1. Field of the Invention
The present invention relates to a lithographic system. More particularly, the present invention relates to concepts of illumination within a maskless lithography system.
2. Related Art
A lithographic system is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic system can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic system, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device). The pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known popular lithographic systems include steppers & scanners. In steppers, each target portion is irradiated by exposing an entire pattern onto the target portion in a single pass. In scanners, each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic system can also be maskless. Maskless lithography, or optical maskless lithography (OML) as known to those of skill in the art, is an extension of conventional (i.e., mask-based) lithography. In OML, however, instead of using a photomask, millions of micro-mirror pixels on a micro-electro-mechanical systems (MEMS) device are dynamically actuated in real-time to generate the desired pattern. MEMS, however, are only one class of OML devices. Due to the fixed grid imposed by the pixels and the use of short-pulse duration excimer lasers at deep ultra-violet (DUV) wavelengths, spatial modulation of gray scales is required. This class of MEMS devices are therefore known as spatial light modulators (SLMs).
In conventional maskless lithography systems, unique challenges are presented with regard to illuminating the SLMs using, for example, excimer lasers. These challenges are that the rays incident on, and reflected off of, the SLM fill the same physical space. This occurrence makes it difficult to provide spatial separation between the illuminator and the projection optics (PO) within the system.
One class of maskless lithography systems uses beam splitters to provide spatial separation between the illuminator and the POs. Beam splitters, however, are undesirable in the application above because of reasons discussed below. Polarizing beam splitters are generally used to project the radiation beam onto the individually controllable elements of the SLM. The radiation beam is projected through the beam splitter twice and a quarter wave plate is used to change the polarization of the radiation beam after a first transmission through the beam splitter and before a second transmission through the beam splitter. Use of polarization to control the direction of the radiation beam means that the cross section of the radiation beam has a uniform polarization. As a result, different polarizations cannot be used to create different effects during the exposure. Also, beam splitters are inefficient and can be difficult to thermally control.
Further still, polarized beam splitters cannot be used, for example, in high numerical aperture (NA) maskless lithography because they do not preserve the polarization state of the light, which is a requirement for high NA maskless lithography optical systems. Non-polarized beam splitters are an alternative. However, non-polarized beam splitters have unacceptably low transmission. Finally, tilted illuminators cannot be used with high NA PO unless the SLM works in “blazing” mode (non-zeroth diffractive order beams enter the PO after reflection off of SLM). This is because of inherent limitations of PO NA for an off-axis pupil.
Therefore, what is needed is a maskless lithography system and method that eliminates the need for beam splitters.